Vacuum distillation of reduced crude



Ot 9, 1951 R. B. SMITH 2,570,607

VACUUM DISTILLATION OF' REDUCED CRUDE I Filed Nov. 18, 1948 READING BARLow SMITH ATTORNEYS UNITED STATES PATENT .OFFICE VACUUMDISTILLATION OF REDUCED 'CRUDE Reading Barlow Smith, Hammond, Ind., assig'nor lto Sinclair Refining Company, New York, N. Y., ia corporation of Maine Application November 18, I948, Serial No. 60,732

v 1 My invention relates to improvements in vacuum distillation of heavy hydrocarbon stocks which are susceptible-to cracking. More particularly, vit is characterized by vacuum operation under higher conditions of temperature and pressure than have been beforeapplied so as to increase .vacuum still capacity, or, conversely, reduce size of vacuum equipment.

rThe immense demands placed upon the petroleum industry by military necessity and mechanization in all phases of modern life have magnified many problems of petroleum refining from problems of efficient and economic operation to problems of productive necessity. The wide spread introduction of s o-called fluid catalytic cracking and other catalytic cracking processes has greatly increased production of high-grade gasoline and heating oils, b ut in turn has placed great pressure upon existing rening means for preparing cracking feed stocks. The preparation of cracking feed stocks is a ready example of the role vacuum distillation plays in rening practice in economically obtaining ultimate yields of useful products from each barrel of crude oil. In'running reduced crudes, for example, in the preparation of cracking feed stocks, the reduced crude charge is conventionally heated in a tube or coil heater and is flashed into a vacuum tower or still from which as much clean distillate or gas oil as possible is taken overhead. In conventional practice it is commonly accepted that the ash temperature and consequently the heater temperature is limited by the temperaturesl at which incipient cracking occurs; that is, about '700 to 800 F. As is well-known, cracking makes gas, and even small quantities of :xed gases in the vacuum lzone seriously interfere with Athe eiciency ofthe vacuum operation. Forjthe maintenance of subatmospheric pressures in the vacuum still requires in practical installations the use of vacuum jets which depend for eiiciency upon substantially complete condensation of the vapor throughput. Thus the presence of small amounts of fixed gases sharply reduces jet eniciency and requires such large and costly jets that operation quickly becomes impractical. Hence, in conventional practice it is customary to set a ash temperature, which when carried back through the system .to the heating zone, will avoid cracking in the system, and then reduce the pressure in the vacuum flash tower to the point necessary for the desired overhead yield. Obviously, the demand for high percentage overhead yields results in flash towers which are disproportionate in size to the throughput.

1 Claim. (C1. 19e-77) vaporization in the heater has been experienced.

This dificulty is generally avoided by holding pressure on the heater coil, thereby preventing vaporization. But, since vaporization is prevented, the temperature runs considerably higher because all of the heat input goes to sensible heat. Furthermore, since the feed is maintained in the liquid phase, the residence time in the heater is increased. The combination of higher temperatures and increased residence time of i course promotes cracking, and accordingly limits the benefits of liquid phase operation in conventional practice.

I have found, however, that if I depart from conventional practice and deliberately establish a higher pressure and temperature in the system, I can significantly increase vacuum still capacity or signicantly reduce vacuum still size by removing fixed gases made in cracking prior to thevacuum step. VI thus avoid interference with the efliciency of the vacuum jets while running under conditions of higher throughput, and provide for improved flexibility of operation with respect to pressure and phase, temperature and percentage overhead yields.

` Where conventional practice is limited to ash temperatures in the neighborhood of 650 to 750 F. and requires a low vacuum still pressure Within the range of 20 to 75 mm., I maintain a pressure Within the approximate range of 150 to 350 mm. of mercury, preferably say about 200 mm., and a temperature within the approximate range of 750 to 850 F., preferably say about 830 F. I find that my improved system of operating results in a reduction in the diameter of the ash still of from one-quarter to one-half or more. To eiect such results, I prefer to operate at a ,heater temperature of about 900 F. and to interpose a relatively small flash drum between the vacuum still inlet and the heater outlet. The pressure on the drum is usually about 90 p. s. i. a., accounting for pressure drop through the heater coil, and the temperature is a few degrees below that of the heater outlet.

I find that my improved system is particularly valuable in running reduced crudes to ultimate gas oil yields in cracking stock preparation for y it greatly increases the capacity of existing equipage and gasoline and heating oil shortages. Conversely, it permits reduction in the size of equipment necessary for the same production, and permits the use of vacuum jets in the most eicient size range and at the most economical cost. Since vacuum jet operation is necessarily inflexible, my improved scheme of vacuumzdistillation introduces ya certain degree of flexibility into a previously inflexible process. I have also found that my improved method eliminates operating difliculties encountered by pushing capacity under the conventional scheme Vof 'operation. For high pressure-high temperature vacuum operation not only brings the 'system into size, but reduces entrainmetof lasphaltic constituents and clears up the color of the gas oil overhead.

An embodiment of my invention is further illustrated in the drawing and the following illustrative example, neither of which is oifered, neuerer, in a ,limitinsisense- In the drawing, charge oil is introduced to the system through line l and is charged to heater l I at a pressure that will maintain 90 p. s. i. a. in flash drum l2.` The heated oil enters flash drum l2 through line i3, and freed from light gases passes to Vacuum still I4 through line l5 and pressure reducingyalve [6. Light gases are taken overhead by means of line IT, and are passed through condenser i8 to receiving drum I9 from whence xed gases are released through line 20 to renery fuel or other purposes. Light gas oil is collected and pumped to the cracking process through line 2|. In vacuum still I4, gas oil is ilashed overhead at 200 mm. of mercury and a temperature of A830" F. through line 22, condenser 23 and receiving drum 24. Line 25 leads to a conventional jet condenser system (not shown) and line 26 serves to remove gas oil to the cracking process. Asphalt .bottoms are removed through line 21.

The operation. of my improved system is perhaps best illustrated `by lcomparison with conventional operation on a unit running 510 barrels per hour of 20 API feed stock. The unit is run to gas oil and asphalt of the following characteristics Gas oz'l Asphalt butts Gravity, API 23.1 Gravity, AP1 9.9 Bb1s./hr 410 Bbls./hr 100 Aniline pt 187 Penetration 211 Carbon res 0.21

A comparison of the feed and overhead is seen! The above rates and product qualities are produced by operation at a flash temperature of rT05" F. and a flash pressure of 36 mm. of mercury. The tower top temperature is 665 F. and the heater eiuent is 748 F. The diameter of the tower necessary for this operation, which is pushing maximum capacity, is 161/2 feet. When changed over-to my improved method by utilizing a flash drum of 4 feet by 10 feet intermediate the heater outlet and the vacuum still, a vacuum tower diameter of only 11 feet is required. The 'rate and product qualities are the same, but the flash temperature is raised to 830 F., the tower `top temperature is .800 F., and the ash prese liquid phase heating. At lower pressures which permit vaporizatio'n the heater eflluent is about 870-to 880 F. Cracking occurs under these conditions although in terms of cracking still operation, .-it is relatively small. However, in terms of the size of jets on the vacuum tower, the amount of gas produced would require such abnormally large j ets ythat the operation would not be feasible. In the described example, however, removal of Vthe gas facilitates the illustrative high temperature and pressure operation.

I claim:

In vacuum distillation of reduced crude oil stocks to prepare gas oil cracking stocks, the steps of heating a reduced crude stock in the liquid phase under superatmospheric, pressure to a temperature of theV order of about 900 F. to effect incipient cracking, passing the treated stock to a relatively small-flash Zone and removing substantially all xed gases while maintaining the stock substantially in the liquid phase under superatmospheric pressure, flashing the preheated andV degassed stock in an enlarged vacuum zone at about '150 toV 850 F. under a pressure within the approximate range of to 350 mm. of mercury, removing vaporized gas oil overhead from the vacuum zone and removing unvaporized bottoms from the vacuum zone.

' READING BARLOW SMITH.V l

RrFEsENQr- S CITED The following references are of record in the file of this patent: i

UNITED STATESVPATEN'I'S Houghland A Aug. 24, 1943 

